California NORML Research Report

Driving Studies Show Limited Accident
Risk from Marijuana

by
Dale Gieringer, Ph.D. - Coordinator California NORML

Scientific studies on marijuana and driving fail to support
the notion that marijuana poses a significant public highway safety hazard,
according to evidence compiled by NORML.

In particular, the evidence fails to support proposals
by marijuana opponents to impose tough new "zero-tolerance" standards
for driving under the influence of marijuana or to disallow on-site use in
medical cannabis clubs out of fear for driving safety.

In general, the evidence shows that marijuana is a lesser
traffic hazard than alcohol or other drugs. Marijuana appears to be
most dangerous in high doses, or when combined with alcohol. On the
other hand, drivers with lower levels of marijuana have been found to be no
more dangerous, and in some cases arguably safer, than other drivers.

NORML has recently issued a comprehensive report
on drugged driving, drug testing, and driving under the influence laws,
by its senior policy analyst, Paul Armentano [1].
The report disputes the rationale for proposed "zero tolerance" laws, in which
the presence of any trace of marijuana in blood or urine is taken as per se
proof of driving under the influence of drugs (DUID). Because marijuana can
be detected in the system long after any impairment has passed, such laws
wrongly misclassify many sober drivers as "intoxicated." Urine tests
detect only non-psychoactive metabolites of marijuana, which linger in the
system for days or even weeks after use. Blood tests can measure the
major psychoactive ingredient of marijuana, THC, which provides a better
but still inexact indicator of recent intoxication. Blood THC
peaks within the first hour of intoxication, but can be detected at lower
levels for a day or more in chronic users.

A growing body of scientific evidence shows that drivers
with modest amounts of THC in their system are no more dangerous than other
drivers. A 2002 review of seven separate crash culpability studies involving
7,934 drivers reported, "Crash culpability studies [which attempt to correlate
the responsibility of a driver for an accident to his or her consumption of
a drug and the level of drug compound in his or her system] have failed to
demonstrate that drivers with cannabinoids in the blood are significantly
more likely than drug-free drivers to be culpable in road crashes"[2].

Two other, new accident studies have failed to find any hazard from marijuana.
A study of 1500+ patients admitted to a Midwest trauma center published in
the Journal of Trauma Injury, Infection, and Critical Care found correlations
between use of alcohol, cocaine and opiates with injuries [3
]. However, their data did not show "any statistically significant
independent associations between injury and cannabis," researchers told
NORML. "(This) offers a strong rebuttal to the Drug Czar's misleading
allegations implying that marijuana is a leading cause of ER admissions,"
says Armentano.

Another study of road trauma from the Netherlands, which detected the presence
of drugs through urine as well as blood tests, found significantly higher
accident risks for alcohol and benzodiazepines (prescription tranquilizers)
and less certain risks for amphetamines, cocaine and opiates, but no increased
risk for cannabis [4].

In the largest U.S. survey of drug use and driving accidents to date, the
National Highway Transportation Safety Administration found that alcohol was
by far the "dominant problem." At the same time it found "no indication that
marijuana by itself was a cause of fatal accidents"[5].
The report was delayed and not publicized because it failed to confirm the
expectations of administration drug warriors.

The NHTSA report did find that the combination of marijuana with alcohol and
other drugs was highly dangerous. Similar results have been reported
in other studies. For this reason, California NORML does not recommend
permitting liquor sales on premises where marijuana is allowed.

On the other hand, studies have found that marijuana by itself tends to be
significantly less dangerous than alcohol. A second NHTSA study of marijuana
on actual driving performance found that the effects of THC appeared "relatively
small" and less than those of drunken driving [6].
It found that marijuana appeared to produce greater caution in drivers, apparently
because users were more aware of their state and ready to compensate for it,
whereas alcohol tended to encourage speeding and risky behavior. However,
it also noted that marijuana could be dangerous in emergency situations that
put high demands on drivers, or in combination with other drugs, especially
alcohol.

Other
studies have shown that at sufficiently high doses marijuana does impair driving
safety. Lab studies have demonstrated noticeable adverse effects for
the first couple hours of intoxication, including impaired attention, unsteady
lane control and following distance, and slower reaction time.

Most
recently, a large-scale Australian accident survey found that drivers with
higher THC blood levels -- particularly those above 5 nanograms per milliliter
(ng/ml) in plasma, indicating that the cannabis use had likely occurred within
the past couple of hours ­ were correlated with a higher accident risk.
However, THC levels below 5 ng/ml were associated with a lower risk than drug-free
drivers [7].

Similar findings have been reported by an expert
panel of the International Association of Cannabis Medicine charged with
developing scientific per-se standard for driving under the influence of marijuana.
In its report, the panel concluded that THC blood plasma levels above 7 -10
ng/ml might reasonably be taken as per se standards of impairment. It
rejected lower blood level standards and standards based on urine metabolites
as unsupported by the scientific evidence. However, it did suggest
that lower THC blood limits might be appropriate where alcohol is also present
[8].

In sum, current scientific evidence shows that the hazards
of marijuana are dose-dependent, but generally less than those of alcohol.
There is no evidence that marijuana has contributed signficantly to overall
accident mortality. In fact, traffic accident mortality rates
declined over the time when marijuana was first popularized in the 60s and
70s, and have continued to decline since.

In light of the evidence, California NORML supports
regulations to allow on-site use of marijuana in designated public outlets,
as presently allowed in the Netherlands. NORML likewise strongly opposes
"zero tolerance" standards for DUID.

8) Franjo Grotenhermen et al, Developing
Science-Based Per Se Limits for Driving under the Influence of Cannabis:
Findings and Recommendations by an Expert Panel" (International Association
for Cannabis as Medicine, 2005).